Within recent years, considerable interest has existed in starch derivatives which are capable of absorbing and retaining large amounts of water. Water-absorbent starches developed at U.S.D.A. Laboratories (e.g., see U.S. Pat. Nos. 3,935,099; 3,997,484; 3,985,616; 3,981,100 and 4,405,387) reportedly absorb more than 1,000 times their dry weight in water. These water-absorbent starches are prepared by grafting polyacrylonitrile to starch molecules in a confined polymerization reactor and thereafter chemically derivatizing the nitriles to carboxyamides and carboxylates. Grafting is typically accomplished by free-radical initiation with ceric ions which remain as a residual contaminant even though the product may be extensively refined. The grafting level is critical, difficult to control and time-consuming.
After the polyacrylonitrile-starch copolymer has been formed, the nitrile groups are saponified to the water-absorbent starch product. The saponification is preferably accomplished by reacting the nitrile groups with an alkaline hydroxide to form carboxylate and carboxyamide groups respectively at a molar ratio of about 2:1. The 66% carboxylate and 33% carboxyamide ratio remains relatively constant. Distribution and sequence of the carboxylate and carboxyamide groups throughout the graft portion follows a regular pattern. Residual heterocyclic nitrogen contaminants (an intermediate reaction product) may be carried into the final product. The water-absorbency properties of the saponified product is significantly improved by purifying the product. Notwithstanding, salt contaminants (as well as the initiators) are difficult to remove and can interfer with the overall water absorbing characteristics of the finished product.
In U.S. Pat. No. 3,640,925 by Touzinsky et al., a process for producing starch graft copolymers is disclosed. In this process, a conventional starch jet cooker operated at superatmospheric pressures and elevated temperatures is used as a copolymerization reactor. An aqueous slurry containing up to 43% by weight starch, comonomer in an amount up to 37% by weight of the starch and free-radical initiator is passed through the starch cooker, rapidly heated to 190.degree.-340.degree. F. to instantaneously paste the starch and to simultaneously initiate the graft copolymerization between the pasted starch and the monomers. After the copolymerization reaction has been completed, the graft copolymerizate may be converted to a dry product. Reportedly a wide variety of acrylic and vinyl monomers may be grafted by this procedure. The need to purify or blanket the copolymerization reactor with nitrogen or exclude air or oxygen (deemed essential to the grafting process) is reportedly avoided. The starch graft copolymers are reportedly useful as wet-end paper and textile sizes. Touzinsky et al. do not contemplate water-absorbent starch products.
British Pat. No. 1,495,845 reports starch and synthetic polymer aqueous mixtures may be roll-dried at elevated temperatures to provide graft polymers which may be used as thickening agents in latices and adhesive dispersions.
A simple and more economical process for producing water-absorbent starch products would represent a significant advance towards the commercial adaptation of these potentially useful products. A process which would eliminate the need for copolymerization and saponification reactors would have a decided advantage over the most highly regarded processes. It also would be a decided commercial advantage to conduct the process in such a manner so as to avoid contamination of the water-absorbent starch products with initiator and saponification salt residues. Greater versatility with respect to the types of monomers copolymerized with the starch and the ability to introduce a predetermined and/or higher level of water-attractant moieties into the synthetic copolymer portion would afford the art the ability to manufacture uniquely different water-absorbent starch products.